The instant invention relates to wastewater treatment, and in particular to processes for the oxidative removal of inorganic thiosulfate from wastewater employing peroxygen compounds.
Several manufacturing processes such as petroleum refining, flue gas desulfurisation, the production of inorganic chemicals, canning processes etc generate waste waters containing undesirable high concentration of thiosulfate compounds. In many instances, the presence of thiosulfates has arisen from the partial air oxidation of sulfides that initially were present. Although it is theoretically possible to air oxidise sulfides beyond the intermediate of thiosulfate, in practice such a procedure is not followed because it is excessively expensive as a result of the greatly enhanced excess air requirements and the concommitant ten-fold increase in the oxidation tower volume requirement. However, the residual thiosulfate from the intermediate air oxidation process still exhibits an immediate chemical and biological oxidation demand and in consequence further treatment is necessary before such wastewater can be discharged into public systems.
Hitherto, one of the chemical reagents proposed for the oxidative removal of thiosulfate from aqueous solutions has been hydrogen peroxide, reference appearing in, for example, a Monograph on Hydrogen Peroxide by Schumb, Satterfield and Wentworth and published by Reinhold Publishing Corporation in 1955, p.399, where it was also reported that various catalysts such as molybdate had been investigated for this reaction. More recently, renewed interest has been shown in the use of hydrogen peroxide for this reaction. A paper by Mucenieks and Raleigh, entitled The Use of Hydrogen Peroxide for the Oxidation of Sulfur Chemical Wastes, presented at the 7th Middle Atlantic Industrial Waste Conference at the Rexel University, 1974 indicated, somewhat discouragingly, that the amount of hydrogen peroxide required to oxidise thiosulfate increases more than eight-fold as the pH of the wastewater increased from pH 6 to pH 9. It will be understood that various of the waste waters containing significant concentrations of thiosulfate are not only alkaline, but contain buffers so that the amount of acid required to neutralize such buffered waste waters to below pH 7 often renders such an acidification process uneconomic and attainment of acidic conditions permits toxic hydrogen sulfide gas to be emitted should the wastewater contain residual sulfide. Accordingly, some alternative method for improving the hydrogen peroxide oxidation process of thiosulfate in alkaline conditions would be of practical benefit. In the context of oxidatively removing thiosulfate from aqueous acidic solutions using hydrogen peroxide, certain transition metal catalysts have been sugqested including molybdenum and tungsten, and vanadium by W. H. Kibble in articles in respectively Plant Engineering, Vol 33 No. 23 pp.137-140 entitled "Treating Industrial Wastewater with Hydrogen Peroxide" and Industrial Water Engineering, Aug-Sept 1976 pp.6-11 entitled "Peroxide Treatment for Industrial Waste Problems". Disadvantageously, though, Kibble also disclosed that a faster reaction rate under acidic conditions by the addition of such catalysts was achieved only at the expense of substantially increased hydrogen peroxide consumption. Such a substantial increase in peroxide consumption militates against contemplating use of a catalyst in alkaline conditions which have already suffered an eight-fold increase in hydrogen peroxide consumption. When comparative trials were carried out using molybdenum as catalyst for hydrogen peroxide in an alkaline system, it was found marginally to increase the rate of thiosulfate removal over addition of the same amount of hydrogen peroxide but catalyst free; for example, in one test after 1 hour the thiosulfate removal had been increased from about 59% to about 62% by addition of the molybdenum catalyst.
There has also been disclosed in U.S. Pat. No. 4,443,342, Stas et al assigned to Interox, a disclosure that divalent copper can be employed to catalyse the peroxide oxidation of dialkyl disulfides, but this specification teaches the use of a pH of from 2.5 to 6.5 for the aqueous medium, preferably from pH 3 to 5, and further teaches the addition of various types of acids in order to reach the desired pH.